Nicola Sturgeon has said the decision by the Supreme Court that the prorogation of the House of Commons was unlawful was of "truly historic proportions" as demands were made for Boris Johnson to resign.
The First Minister said the Supreme Court had upheld the judgement of the Court of Session that the prorogation had "no effect", that Boris Johnson should resign and that if he did not go voluntarily, "parliament should come together" to force him from office.
Ms Sturgeon said: "We often hear hyperbole at moments like this but it's no exaggeration today to say this is the most significant and historic constitutional court ruling that we've had in all of our lifetimes.
"To be blunt it's found the Prime Minister acted unlawfully in seeking to prorogue Parliament, that he had no good reason for doing so, that in fact it was about evading scrutiny and closing down parliamentary accountability, and all at a time of political and constitutional crisis for the country.
"I think what should happen now is that Parliament should be back in session and, I don't say this lightly, but I think Boris Johnson should resign."
She added: "This is a Prime Minister who has been found to have acted unlawfully, seeking to evade scrutiny and without good reason and if the rules of democracy are to mean anything, a Prime Minister with any honour would tender his resignation today.
"This is about the conduct and behaviour of a Prime Minister who is not fit for office. Parliament's principle priority should continue to be to block a no deal Brexit at the end of October - but if the PM isn't prepared to do the decent and honourable thing and tender his resignation, then Parliament should quickly come together to force this Prime Minister from office.
"This is the most significant court judgement we have ever seen, it is quite extraordinary and the manner of its delivery was quite extraordinary
"If he is able to stay in office then the precedent that sets and what that means for democracy and accountability in the future is pretty grim."
Asked if the Prime Minister had ever lied to her, Ms Sturgeon said she had only had one meeting with Mr Johnson in he was "dismissive" about the concerns around a no-deal Brexit. She said he was "not being frank" about the implications of such and added: "I know that he and his government are trying to underplay the severity [of a no-deal Brexit] and has not been straight with the people of the UK let alone politicians.
"In terms of the health of our democracy we cannot allow a Prime Minister who has been the subject of that extraordinary ruling to remain in office with impunity - that would be the wrong decision."
The First Minister also tweeted a photograph of her Cabinet in Bute House watching television as Lady Hale made the dramatic announcement that the government decision was "unlawful", and called it a ruling of "truly historic proportions".
She congratulated the petitioners, who took the case against the UK government, including Edinburgh SNP MP Joanna Cherry, who said after the judgement that Parliament must "reconvene without delay".
Ms Cherry said: "I am delighted the UK Supreme Court has followed Scotland’s supreme court and ruled that Boris Johnson's undemocratic decision to shut down Parliament ahead of Brexit was unlawful and unconstitutional.
"Boris Johnson should resign. His behaviour has been disgraceful and his position is untenable - if he had a shred of integrity he would jump before he is pushed.
"Parliament must resume without delay, so we can hold the Tory government to account on its Brexit plans, which threaten to plunge the UK into recession, destroy 100,000 Scottish jobs, and inflict lasting harm on living standards, public services and the economy."
The SNP's Westminster leader, Ian Blackford added: "I'm delighted the Supreme Court has done what it has done today. We need to get back to work and hold this government to account. Boris Johnson should now resign.
"There was only one reason the Prime Minister wanted to shut down Parliament, because he doesn't have a majority. This is a big day for democracy."
Scottish Labour MP Ian Murray, another of the petitioners, said: "This is a historic result. The courts have upheld British democracy and delivered an astonishing rebuke to Boris Johnson for his disgraceful behaviour .
"The Prime Minister lied and politicised the Queen. He now has no option but to resign.
"It was a scandal that he suspended Parliament at the height of the biggest political crisis this country has faced since the Second World War. MPs must now be allowed to get back to work as early as tomorrow, and by working together we can solve this crisis by giving people a final say on Brexit - with the option to remain in the EU."
Scottish Conservative MSP, Adam Tomkins, his party's constitution spokesperson, said the government now had to obey the court's decision.
Taking to Twitter he said: "In a constitutional clash between the executive and parliament, this is a Supreme Court that will back parliament. So be it. The executive must obey the law."
But he added: "Important to note that it is the *effect* of the order to prorogue on which the Court has focused, not the PM’s purpose or motivation. This is *not* about the PM “lying to” HM the Queen.
"However you cut it it’s a stunning judgement, and an emphatic reassertion of the fundamentals of our seventeenth-century constitution: the Crown’s government is subject to the will of Parliament, and not the other way around."
Scottish Greens Co-Leader Patrick Harvie MSP added to the calls for Mr Johnson's resignation “In light of the clear judgement of the Supreme Court that the decision to prorogue Parliament was unlawful, void and of no effect it’s clear that the Prime Minister and his Cabinet must resign immediately," he said.
"Their reckless actions demonstrated complete contempt for parliamentary democracy. Parliament must be recalled at once in order to hold this out of control regime to account.”
Litigation partner at Pinsent Masons, Jim Cormack QC, said the Supreme Court judgement could now make any further prorogation of Parliament difficult for the government.
He said: "As the political shock waves of today’s decision by the Supreme Court continue to reverberate, the government will take little comfort from the fact that the court has neatly side-stepped whether the motive for this prorogation had been truly stated by Government or not.
"However, by focusing on the effects of prorogation, the Supreme Court has restricted the Prime Minister making it very difficult for him to attempt to prorogue again in the run up to the Brexit deadline.
"In the eyes of the courts he could very well be seen to be acting unlawfully on a second occasion by interfering with parliamentary scrutiny without a clear justification during an unprecedented and critical period for the future of the UK."